What is an irregular cycle?

Although some variability in cycle length is to be expected, a cycle is considered irregular when most of your cycles in the previous six months out of range. If the gap between your periods is changing a lot, you’re experiencing an irregular cycle.

What does this mean?

This could mean your periods happen infrequently or are very unpredictable. For adolescents (ages 10 – 19) [1], cycles outside of the range of 21-45 days are considered irregular [2]. For adults, cycles outside the range of 24-38 days [3]. Another indication is if periods suddenly stop for no apparent reason (see exceptions below). Doctors usually will want to investigate if you haven’t had a period for 90 days (roughly 3 months) [3].

Why irregular cycles happen

There are a lot of reasons as to why a person experiences an irregular cycle, often it could be an indication of more serious health issues. It’s not always a cause for concern but definitely worth paying attention to. Here’s some common reasons:

Stages of life

  • Cycles can take a few years to regulate after the first period or ‘menarche’
  • As cycles come to an end during perimenopause


  • Pregnancy
  • After a pregnancy (postpartum)
  • Breastfeeding
  • Recurrent miscarriages [4]

Medical conditions

  • Polycystic ovary syndrome (PCOS)
  • Uterine polyps and fibroids
  • Certain cancers can disrupt the menstrual cycle as can benign tumours
  • Early menopause
  • Abnormalities of the cervix or vagina
  • Thyroid disorders
  • Unmanaged diabetes


  • Sleep disorders
  • Jet lag/long-distance travel [5, 6]
  • Intensive exercise
  • Rapid weight changes
  • Eating disorders
  • Birth control
  • Other medications [7 – 10]

When should I see a doctor?

You probably don’t need medical advice if you have always had slightly irregular periods (remember it’s all about finding your normal!), or if you are still going through puberty. However, if you are worried about irregular cycles or wants some answers, consult your doctor. Even if only for peace of mind.

Some reasons to consult a doctor [11]:

  • your periods have suddenly become irregular without any explanation (see examples above) and you’re under 45
  • your periods last longer than 7 days
  • you have irregular periods and are struggling to get pregnant
  • there is big difference between your shortest and longest menstrual cycle
  • if you are bleeding outside of your period

Further reading:


  1. Adolescent health, Who.int
  2. Hillard PJ. Menstruation in adolescents: what do we know? And what do we do with the information?. J Pediatr Adolesc Gynecol. 2014 Dec;27(6):309-19.
  3. Fraser IS, Critchley HO, Broder M, Munro MG. The FIGO recommendations on terminologies and definitions for normal and abnormal uterine bleeding. Semin Reprod Med. 2011 Sep;29(5):383-90.
  4. Griebel CP, Halvorsen J, Golemon TB, Day AA. Management of spontaneous abortion. Am Fam Physician. 2005 Oct 1;72(7):1243-50.
  5. Mahoney MM. Shift work, jet lag, and female reproduction. International journal of endocrinology. 2010 Mar 8;2010.
  6. Baker FC, Driver HS. Circadian rhythms, sleep, and the menstrual cycle. Sleep medicine. 2007 Sep 30;8(6):613–22.
  7. Hatch MC, Figa-Talamanca I, Salerno S. Work stress and menstrual patterns among American and Italian nurses. Scandinavian journal of work, environment & health. 1999 Apr 1:144-50.
  8. De Souza MJ, Toombs RJ, Scheid JL, O’Donnell E, West SL, Williams NI. High prevalence of subtle and severe menstrual disturbances in exercising women: confirmation using daily hormone measures. Hum Reprod. 2010;25(2):491-503.
  9. Poyastro Pinheiro A, Thornton LM, Plotonicov KH, Tozzi F, Klump KL, Berrettini WH, Brandt H, Crawford S, Crow S, Fichter MM, Goldman D. Patterns of menstrual disturbance in eating disorders. International Journal of Eating Disorders. 2007 Jul;40(5):424-34.
  10. Becker U, Tønnesen H, Kaas-Claesson N, Gluud C.Menstrual disturbances and fertility in chronic alcoholic women. Drug Alcohol Depend. 1989 Aug;24(1):75–82.
  11. Irregular periods, nhs.uk